A fuel cell is an energy conversion device that converts chemical energy into electrical energy. The fuel cell generates electricity and heat by electrochemically combining a gaseous fuel, such as hydrogen, carbon monoxide, or a hydrocarbon, and an oxidant, such as air or oxygen, across an ion-conducting electrolyte. The fuel cell generally consists of two electrodes positioned on opposite sides of an electrolyte. The oxidant passes over the oxygen electrode (cathode) while the fuel passes over the fuel electrode (anode), generating electricity, water, and heat.
A solid oxide fuel cell (SOFC) is constructed of solid-state materials, utilizing an ion conductive oxide ceramic as the electrolyte. The electrochemical cell in a SOFC is comprised of an anode and a cathode with an electrolyte disposed therebetween. The components of an electrochemical cell and a SOFC are rigid and extremely fragile since they are produced from brittle materials.
In a SOFC, a fuel flows to the anode where it is oxidized by oxygen ions from the electrolyte, producing electrons that are released to the external circuit, and mostly water and carbon dioxide that are removed in the fuel flow stream. At the cathode, the oxidant accepts electrons from the external circuit to form oxygen ions. The oxygen ions migrate across the electrolyte to the anode. The flow of electrons through the external circuit provides for consumable or storable electricity. However, each individual electrochemical cell generates a relatively small voltage. Higher voltages are attained by electrically connecting a plurality of electrochemical cells in series to form a stack.
The SOFC stacks exhaust unused fuel and oxidant. This exhaust is utilized in a waste energy recovery unit as a source of chemical and thermal energy. The waste energy recovery unit is a device that converts chemical energy and thermal energy into input thermal energy. This is accomplished with heat exchangers. However, waste energy recovery units can be inefficient in recovering the waste energy, and allow emissions of undesirable gaseous compounds, due to incomplete reactions of the unused fuel and oxidant.
The drawbacks and disadvantages of the prior art are overcome by a method and apparatus for a waste energy recovery assembly for a fuel cell system, which are disclosed. In one embodiment, the waste energy recovery assembly for a fuel cell system comprises: a cathode exhaust passage in fluid communication with a mixing zone through a collection chamber; an anode exhaust passage in fluid communication with said mixing zone by way of an anode exhaust gas orifice disposed in a direction capable of forming an anode exhaust gas flow entering said mixing zone at an angle of about 45xc2x0 to about 135xc2x0 to a combined gas flow entering said mixing zone from said collection chamber; a thermal exchange structure in fluid communication with said mixing zone; and an exhaust gas passage in fluid communication with said thermal exchange structure.
One embodiment of the method of using a waste energy recovery assembly for a fuel cell system comprises: supplying reformate and supply air to said waste energy recovery assembly; directing said supply air and said reformate through a thermal exchange structure in said waste energy recovery assembly, wherein said supply air is separated from said reformate; introducing cathode exhaust gas to a mixing zone; introducing anode exhaust gas to said mixing zone at an angle of about 45xc2x0 to about 135xc2x0 a combined gas flow direction to form composite gases; combusting said composite gases; and flowing said combusted composite gases through said thermal exchange structure in thermal communication with said reformate and said supply air.